Point users to peps.python.org/release-schedule.ics #72

Thanks to python/peps#4705 there is now https://peps.python.org/release-schedule.ics

We should point our users to it. GitHub pages does not support HTTP redirects, so we can:

• document the new location in readme
• copy the official ics instead of scraping HTML
• include deprecation notices in even descriptions
• later: include deprecation notices in event names